Switching device for telecommunications channel

ABSTRACT

A switching device for a telecommunications channel. The device comprises a voltage detector, a switch, and a storage device. The voltage detector produces an output voltage if the voltage between the lines of the channel exceeds a predetermined level, for example 110 volts. The switch can increase an impedance in one of the lines (preferably causing open circuit) in response to an input voltage. The first storage device, for example a capacitor, produces the input voltage when it reaches a charge V 2  ; is initially at a voltage V 1 , for example zero, that is less than V 2  ; after charging by the output voltage from the voltage detector, is at a voltage V 3  greater than V 2  ; and discharges from V 3  to V 1  in the absence of the output voltage. The voltage detector comprising a low-pass filter such that an AC signal on the channel produces an output voltage that charges the first storage device to a charge below V 2 .

FIELD OF THE INVENTION

This invention relates to a switching device, particularly a maintenancetermination unit (MTU) for use in a telephone network.

BACKGROUND OF THE INVENTION

Recent de-regulation of the telecommunications industry has resulted ingreater freedom for subscribers to install their own equipment, and as aresult telephone companies have insisted that their responsibility formaintenance of the network end at the subscribers' premises. It hastherefore become necessary for clear demarcation points to beestablished and for the telephone companies to be able to determine,preferably remotely, whether any fault lies on the telephone company'sor on the subscriber's side of the demarcation point. In order to dothis, a so-called maintenance termination unit is installed, generallyin or on a subscriber's premises at that point where the telephone cableenters. The intention is that the telephone company can send a specialsignal along the line to discover not only where any fault lies but alsowhat kind of fault exists, for example whether the fault is an open orshort circuit. An MTU is preferably able to disconnect one or both ofthe lines to the subscriber and also to connect those lines together toallow a loop-back test to be carried out. It is clearly necessary thatsuch disconnection and testing be carried out only when desired and not,for example, in response to a ringing signal or other normal linevoltage. Some form of filtering is therefore desirable. Also, it isdesirable that the system be fail-safe such that failure of the MTU doesnot prevent normal operation of the telephone or other subscriberequipment. The present invention therefore provides a particularlyadvantageous form of filtering, and in a preferred embodiment, employsdepletion mode field effect transistors (FETs) as series switches in thelines which will remain conducting in the presence of a fault in theircontrol circuits.

Various designs of MTU have been proposed, of which the following may bementioned.

GB 2149274 (Teradyne) discloses a pulse-operated remote isolation devicecomprising normally conducting field effect transistors together withdetection circuitry that detects a 130 volt command pulse from thetelephone company that activates a control circuit which, by means of atransformer circuit, changes the state of the FETs from a low to a highimpedance. In this way, the subscriber's equipment is disconnected.After a fixed period of 16 to 18 seconds, which is determined by a pulsedetection timer, the control circuit allows the FETs to return to theirconductive state. The circuit required is rather complicated andinvolves a special counter chip producing an AC output which is fed to atransformer isolating circuit, an output of which is then rectified toprovide a control voltage to a chip containing the FETs.

U.S. Pat. No. 4323799 (King) discloses an impulse activated switchhaving a controlled time-delayed self-restoration from its complimentarystate to its initial state. It comprises a photo-diode array connectedto gate and source electrodes of a FET to cause the FET to switch. Acapacitor that forms part of an RC timing network is also charged by thediode array.

U.S. 4710949 (Ahuja) and GB 2097632 (Phillips) may also be mentioned.

Many of the circuits of the prior art are complicated and require largenumbers of electronic components and are consequently likely to beexpensive, large and potentially unreliable.

We have now designed an improved MTU in which a timing circuit fordelayed reconnection of a subscriber can act as a back-up filter to aprimary filter that prevents a normal ringing signal on the line fromactivating the MTU. Whilst this back-up feature might not be requiredunder perfect conditions, it can allow for greater tolerance incomponent characteristics, it can allow for initially ideal componentsto deteriorate with age or to operate outside expected temperatures andit can reject spurious line voltages.

SUMMARY OF THE INVENTION

Thus, the prevent invention provides a switching device for a channelcomprising a pair of lines such as a telecommunications channel whichcomprises:

(i) a voltage detector that produces an output voltage in response to avoltage between the lines above a pre-determined level, preferably about110 volts;

(ii) a switch, that can change an impedance (preferably increase it) inone of the lines (preferably causing open circuit) in response to aninput voltage thereto; characterized in that:

(a) the device additionally comprises a capacitor or other first storagedevice that produces said input voltage, said input voltage beingproduced when a storage device reaches a charge V₂, said storage devicebeing initially at a voltage V₁ (which is preferably zero) that is lessthan V₂ and after charging by the output voltage being at a voltage V₃that is greater than V₂, said storage device discharging from V₃ to V₁in the absence of the output voltage; and

(b) the voltage detector (1) comprises a low-pass filter such that an ACsignal on the channel produces an output voltage that charges the firststorage device to a charge below V₂.

We prefer that the voltage detector produce its output voltage inresponse to a DC voltage of at least, say, 110 volts between the lines.It may be desirable that the switches be electrically isolated from thevoltage detector. We prefer that this be achieved by the use of anopto-electronic device: when the voltage between the lines reaches thepre-determined level a light emitting diode (LED) becomes activatedwhich in turn illuminates a light activatable cell whose output voltagecharges the charge storage device. Where a switch is provided in eachline of the communications channel, a single LED may power twolight-activatable cells one for each switch. In this way each switch isisolated from the other and from the voltage detector. The voltagedetector preferably gives rise to said output voltage when a currentflows through it, this preferably causing a capacitor to charge. Thiscapacitor may be connected between the gate and the source of a FET inorder to bias it into its conductive state when the pre-determinedvoltage between the lines of the communications channel produces thepassage of current through the voltage detector. An LED may be connectedin series with this FET, again between the lines of the communicationschannel. A further circuit may be provided by means of which thecapacitor may discharge. The circuit may be configured such thatsufficient charge builds up on the capacitor only when a DC or lowfrequency voltage above a certain value is applied to the lines. At highfrequencies, any charge built up on the capacitor during one part of thevoltage cycle will be dissipated in a later part. In this way, thevoltage detector provides a filtering effect and will not produce saidoutput voltage in response to a ringing signal on the line.

It may happen, due to ageing of components etc., that a ringing signalwhich should be ignored by the voltage detector in fact producessufficient charge on the capacitor that the FET is switched. If theerror is small the FET will be switched for only a small part of thecycle of the ringing signal, and the LED too will emit light for only asmall part of each cycle. In turn the light-activatable cell willproduce a brief output which will charge the first storage device. Thefirst storage device will, however, be charged only to a charge belowV₂. This level of charge is of course insufficient for the switch toopen or otherwise to change the impedance in the line. In this way, thefirst storage device acts as a back-up filter to that provided by thevoltage detector.

The first storage device is preferably connected between the gate andthe source of a FET which constitutes the switch, and in preferredembodiments that switch is provided in series in the line. In the mostsimple embodiments the switch is a depletion mode FET since such a FETis normally on and is switched off when a threshold voltage is appliedbetween its gate and source. An advantage of depletion mode FETs is thatunder normal conditions none of the control circuitry need be active.The control circuitry will therefore be used only during testing andwill therefore have a long lifetime. A more complex arrangement couldhowever be produced involving an enhancement mode FET having a voltageapplied between its gate and source during normal operation, whichvoltage is removed in response to said output voltage. Other circuitscould be used employing JFETs or bipolar transistors etc.

We prefer that the switching device be able to be positioned in eitherline (or in both lines either way around) and therefore that it be ableto operate properly with voltages of either polarity. This can beachieved by providing as the switch two FETs, preferably depletion-modeFETs, connected back-to-back, preferably with their sources connectedtogether. Such an arrangement can be particularly advantageous. Forexample it can be current limiting for either polarity, at say 200 mA.Also, the arrangement has a linear I-V characteristic with zero voltageburden, and can readily supply the 20 mA or so required for operation ofa telephone circuit.

The switching device may include various further features. For example ashunt switch may be provided between the lines or between one or both ofthe lines and earth. Such a shunt switch may be able to switch to a lowimpedance in response to a voltage, such as a voltage of similarmagnitude but opposite sign to that to which the voltage detectorresponds. Thus, a subscriber may be disconnected from a central exchangeand a loop back test be carried out to test continuity of that part ofthe network which is the responsibility of the telephone company.

The switching device may also include a protection circuit to protectthe central office and/or the subscriber and/or the MTU from anovercurrent and/or an overvoltage. In a preferred embodiment anovercurrent in one of the lines causes the switches in the lines tobecome open circuit.

The present invention is further illustrated with respect to theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a first embodiment of the invention;

FIG. 2 shows the effect of a ringing signal on the lines; and

FIG. 3 shows the effect of an activation signal on the lines.

DETAILED DESCRIPTION OF THE DRAWINGS

A switching device 1 of the invention is shown interconnecting atelephone central office 2 and a subscriber 3 at a demarcation pointadjacent the subscriber's premises. The switching device 1 has terminals4, 5, 6 and 7 for connection respectively to the tip line input, ringline input, tip line output and ring line output.

The switching device 1 has been shown subdivided into three parts 8,9,and 10, although they need not of course be physically separated in theway drawn. Part 8 is a voltage detector, and part 9 is switch in each ofthe tip and ring lines. Part 10 is an optional loop-back circuit.

The voltage detector 8 works as follows. Diode D1 will be conductive fora positive voltage on the tip line, and diode D2 will be conductive ifsuch a voltage is above its threshold voltage, preferably about 110volts. A positive voltage on the tip line will therefore chargecapacitor C1. Capacitor C1 can however discharge via diode D3 andresistor R2, and as a result the voltage detector can act as a low passfilter. When a ringing voltage is applied between the tip and ringlines, the voltage peaks of which exceed the ringing cycles exceedingthe threshold voltage of diode D2, capacitor C2 will charge for part ofthe ringing cycle. It will however discharge once the voltage of theringing signal has fallen below the threshold voltage of diode D2.Whether or not charge builds up on capacitor C2 over consecutive ringingcycles will depend upon the time constant of circuit R1/C1 compared tothe time constant of circuit C1/D3/R2. In order to avoid activation ofthe voltage detector by means of a ringing signal the time constant ofcircuit R1/C1 should be larger than that of circuit C1/D3/R2. Sincethese circuits have capacitor C1 in common, the time constants willdepend on the values of resistors R1 and R2 and therefore we prefer thatR1 has a higher resistance than R2. Preferably the resistance of R1 isfrom 2-4 preferably about 3, times the resistance of R2, and preferredvalues are about 10 MΩ for resistor R1 and about 330 KΩ for resistor R2.Capacitor C1 preferably has a capacitance of about 330 nf.

When a DC, or sufficiently low frequency, activation signal ofsufficient voltage and of the correct polarity is applied between thetip and ring lines, capacitor C1 will charge to a level sufficient toprovide a threshold bias voltage between the gate and source of FET Q1.Q1 is an enhancement mode FET and the effect of this bias will be toturn it on. Current will now be able to flow via a current-limitingresistor (of about 20 KΩ from the tip to ring lines via circuitD1/D4/Q1. D4 is an LED that forms part of opto-electronic device 11. Theresult of the activation signal is therefore emission of light from LEDD4. This light illuminates optically- active cells D5 and D6 producingoutput voltages on lines 12A/13A and 12B/13B. A diode is shown connectedacross capacitor C1 to provide protection for FET Q1.

These output voltages charge capacitors C2 that are connected betweenthe gate and source electrodes of the FETs constituting the switches inthe lines. In the embodiment illustrated a pair of source-connected FETsQ2 is connected in each line. These FETs Q2 are preferably depletionmode FETs having a threshold voltage of V₂. The output voltages from theopto-electronic device 11 of the voltage detector 8 charge capacitors C2from their initial charging level V₁ (preferably 0), that is less thanV₂, and after charging the capacitors are at a voltage V₃ that isgreater than V₂.

After the initial activation voltage has been withdrawn the output fromthe voltage detector 8 ceases and capacitors C2 discharge from voltageV₃ down to voltage V₁. The FETs Q2 will therefore be switched off by theactivation voltage, and they will remain off during the period overwhich the capacitors C2 discharge from voltage V₃ down to voltage V₂.The length of this period can be selected by suitable choice ofcapacitors C2 and of resistors R3 through which they discharge. Thetiming constant of this circuit C2/R3 will be chosen to be sufficientlylarge that all necessary testing can be carried out by the centraloffice before capacitor C2 has discharged to voltage level V₂. It willalso be chosen to be sufficiently small that the subscriber is notdisconnected from the central office for longer than necessary. It canbe seen therefore that reconnection of the subscriber is automatic thusavoiding the need for the tester to remember to make a reconnection.

Diodes D7 are present to prevent capacitors C2 discharging via theoptically-active cells D5 and D6.

The optional shunt-switch may comprise a triac 14 activated by a secondvoltage detector similar to voltage detector 8, but preferably connecteddirectly instead of via an electrically-isolating opto-electronic device11.

The second voltage detector is preferably activated by a voltage betweenthe lines of opposite polarity to that which activates voltage detector8. This can be seen to be achieved by diodes D8 and D9 which passcurrent when the ring line is made positive with respect to the tip lineat a voltage at least equal to the threshold voltage of diode D9, whichis preferably about 110 volts. As in the case of the first-mentionedvoltage detector, current will flow through R6 to charge capacitor C3 onreceipt of a DC, or low frequency activating voltage of the rightvoltage and polarity. Capacitor C3, when sufficiently charged, providesa threshold bias between the gate and source electrodes of FET Q3 thusturning it on. Current flow through FET Q3 provides the input to triac14 necessary to turn it on. As a result the tip and ring lines becomeinterconnected allowing loop-back tests to be carried out by the centraloffice before FET Q3 switches off as a result of capacitor D3discharging through D10/R7.

FIG. 2 shows two voltage plots on two aligned horizontal time axes. Thetop plot shows a sinoisoidal ringing signal applied between tip and ringlines, whose peak voltage is somewhat greater than the 110 voltsthreshold of diode D2 of FIG. 1. Since the ringing signal at some pointsduring its cycle exceeds the threshold voltage of this diode, capacitorC1 in FIG. 1 will begin to be charged. This is shown in the lower plotof FIG. 2. During the period in which the ringing signal is greater than110 volts capacitor C1 is charged and as the ringing signal dips below110 volts diode D2 will cease to conduct and the charge on capacitor C1will discharge through resistor R2. The lower plot of FIG. 2 shows thatthe charge on capacitor C1 which provides the gate source voltage acrossFET Q1, never reaches the threshold value (V_(GTH)) required for Q1 tobe switched on. The time constant of circuit C1/D3/R2 is smaller thanthat circuit R1/C1 and as a result the charge on C1 dissipates morequickly than it was accumulated, and as a result charge does not buildup on C1 from one cycle of the ringing signal to the next. In this waythe detection circuit 8 acts as a low-pass filter since it is only DC orlow-frequencies that produce an output from the opto-electronic device11.

FIG. 3 shows what happens if the characteristics of the components ofthe voltage detection circuit are not optimum or if the ringing signalhas a lower frequency or greater voltage than expected. FIG. 3 consistsof three plots again with aligned time axes. In the uppermost plot anactivation signal of voltage greater than the 110 threshold voltage ofdiode D2 is applied at time T₀, At time T₁ the voltage has risen to 110volts at which the voltage across capacitor C1 Will begin to rise as isshown in the middle plot. When the voltage across this capacitor hasreached V_(GTH), the threshold voltage of FET Q1, that FET will switchon. This occurs at time T₂. The opto-electronic device 11 will thenproduce the output voltage that charges capacitors C2. By time T₃ theywill have charged sufficiently to produce voltage V_(GTH), the thresholdvoltage-of FETs Q2. This is shown in the lower-most plot to occur attime T₃.

The line switches will therefore switch open after T₃ -T₀ has elapsed.If during this period the activation voltage between the lines dropsbelow 110 volts the switches will not open. The capacitors C2 thereforeprovide a back-up filtering to that of the voltage detector 8 as isrepresented by the time difference T₃ -T₂.

Various tests were performed on the circuit of FIG. 1, giving thefollowing results.

    ______________________________________                                                       Using Siliconix                                                                            Using Siemens                                     Transparency Test                                                                            ND20206      B55149 0.35 V                                     ______________________________________                                        1.  Voltage drop under                                                                           1.96 V       0.35 V                                            operational conditions                                                    2.  Current leakage under                                                                        0.023 μA/4.4 × 10.sup.9                                                           0.05 μA/2 × 10.sup.9                     no load, insulation                                                                          ohms         ohms                                              resistance.                                                               3.  Subscriber loss                                                                              MTU disconnects.                                                                           MTU disconnects.                              4.  Symmetry loss  >65 dB       >65 dB                                        5.  Return loss of 34 dB        42 dB                                             input impedance                                                           6.  Insertion loss in the                                                                        0.24 dB      0.06 dB                                           telephony voice band                                                      7a. Distortion during signal                                                                     <0.1% at 1 KHz                                                                             <0.1% at 1 KHz                                    transmission                                                              7b. Distortion during                                                                            <0.1% at 1 KHz                                                                             <0.1% at 1 KHz                                    signal reception                                                          8.  Return loss of 23 dB        37 dB                                             input impedance                                                           9.  Insertion loss at 12KHz                                                                      0.63 dB      0.11 dB                                       10. Return loss of input                                                                         19 dB        40 dB                                             impedance in OVB                                                          11. Insertion loss in OVB                                                                        0.95 dB      0.18 dB                                       12. Call signal (ringing) loss                                                                   <0.5 V       <0.5 V                                        13. Measure of input                                                                             <0.2 V       <0.2 V                                            impedance                                                                 ______________________________________                                    

The transparency tests were carried out using Keithley 228Avoltage/current sources, Keithley 195 DMMs, a HP8165, a HP8165A signalsource, and a WG SPM12 level meter.

I claim:
 1. A switching device for connection to a channel, the channelcomprising a pair of lines, wherein said switching device comprises:(1)a voltage detector which, in use, produces an output voltage in responseto a voltage between the lines being above a first predetermined voltagelevel, and which comprises(a) a low pass filter; (b) a transistor,which(i) if it is a bipolar transistor, comprises a base, a collectorand an emitter, and (ii) if it is a field effect transistor (FET),comprises a gate, a source and a drain; and (c) a second charge storagedevice connected to the base or gate of the transistor;wherein currentflow through the voltage detector charges the second charge storagedevice thereby biasing the gate or base of the transistor, thus allowingcurrent to flow through the transistor, the current through thetransistor giving rise to the output voltage; (2) a switch having aninput which is connected to an output of the voltage detector, andwhich, in use, is connected in series in one of the lines, and canchange an impedance in the line in response to an input voltage thereto;and (3) a first charge storage device which, in use:(i) produces saidinput voltage when a storage device voltage in the storage devicereaches a voltage level V₂, (ii) initially has a storage device voltagelevel V₁ which is less than V₂, and, after charging by the outputvoltage from the voltage detector, has a storage device voltage level V₃which is greater than V₂, and (iii) discharges from V₃ to V₁ when theoutput voltage ceases;wherein an AC signal on the channel causes the lowpass filter in the voltage detector to produce an output voltage thatcharges the first storage device to a voltage level below V₂.
 2. Adevice according to claim 1, in which the voltage detector allowspassage of current therethrough only when the voltage between the linesexceeds a predetermined minimum voltage.
 3. A device according to claim1, the voltage detector comprising:(1) a first opto-electronic device;and (2) a second opto-electronic device;wherein the current through thetransistor flows through the first opto-electronic device therebygenerating an optical signal which causes the second opto-electronicdevice to generate the output voltage.
 4. A device according to claim 1in which the low pass filter comprises a second charge storage devicewhich acquires charge during part of a ringing cycle, which acquiredcharge is discharged during the remainder of the ringing cycle.
 5. Adevice according to claim 4, in which the charge acquired by the secondcharge storage device during part of the ringing cycle is sufficient tocause said output voltage, said output voltage charging the firststorage device to a voltage below V₂.
 6. A device according to claim 1,in which the switch comprises an FET having a gate, a source and adrain.
 7. A device according to claim 6, in which the switch comprisestwo source-connected FETs.
 8. A device according to claim 6, in whichthe FET is a depletion-mode FET.
 9. A device according to claim 6, inwhich the FET has its source and drain connected in series with theline.
 10. A device according to claim 6, in which the first chargestorage device comprises a capacitor connected between the gate and thesource of the FET.
 11. A device according to claim 1, comprising asecond switch, the second switch having an input which is connected toan output of the voltage detector, the second switch connected in seriesin the other of the lines, and which, in use, can change an impedance inthe other of the lines in response to an input voltage thereto.
 12. Adevice according to claim 1, in which the low-pass filter comprises acircuit including a capacitor, the capacitor being arranged to chargethrough a first circuit path and to discharge through a second circuitpath, thereby causing the time constant for charging the capacitor to begreater than that for discharging the capacitor.
 13. A device accordingto claim 1, in which the first storage device comprises a circuitincluding a capacitor, the capacitor being arranged to charge through afirst circuit path and to discharge through a second circuit path,thereby causing the time constant for charging the capacitor to be lessthan that for discharging the capacitor.
 14. A device according to claim1, comprising a shunt switch which, in use, is connected between thelines, or between one or both of the lines and ground, and which can becaused to switch from a high impedance to a low impedance in response toa voltage between the lines.
 15. A device according to claim 14, inwhich the voltage detector responds to a voltage between the lines ofonly a first polarity, and the shunt switch responds to a voltagebetween the lines of only the opposite polarity.
 16. A device accordingto claim 1, in which the switch will produce a high impedance in one ofthe lines in response to an overcurrent in that line.
 17. A deviceaccording to claim 1, having a switch in each line, a single said chargestorage device producing an input voltage to each of the switches.
 18. Adevice according to claim 1, in which the switch comprises a depletionmode FET having one or more of the following characteristics:(1) avoltage rating from 12 v to 1000 v; (2) an on resistance from 1 Ω to1000 Ω; and (3) a maximum current rating from 20 mA to 1500 mA.
 19. Acircuit comprising(A) a channel, the channel comprising a pair of lines,and (B) a switching device, the switching device comprising:(1) avoltage detector which produces an output voltage in response to avoltage between the lines being above a first predetermined voltagelevel, and which comprises(a) a low pass filter; (b) a transistor,which(i) if it is a bipolar transistor, comprises a base, a collectorand an emitter, and (ii) if it is a field effect transistor (FET),comprises a gate, a source and a drain; and (c) a second charge storagedevice connected to the base or gate of the transistor; wherein currentflow through the voltage detector charges the second charge storagedevice thereby biasing the gate or base of the transistor, thus allowingcurrent to flow through the transistor, the current through thetransistor giving rise to the output voltage; (2) a switch having aninput which is connected to an output of the voltage detector, and whichis connected in series in one of the lines, and can change an impedancein the line in response to an input voltage thereto; and (3) a firstcharge storage device which:(i) produces said input voltage when astorage device voltage in the storage device reaches a voltage level V₂,(ii) initially has a storage device voltage level V₁ which is less thanV₂, and, after charging by the output voltage from the voltage detector,has a storage device voltage level V₃ which is greater than V₂, and(iii) discharges from V₃ to V₁ when the output voltage ceases;wherein anAC signal on the channel causes the voltage detector to produce anoutput voltage that charges the first storage device to a voltage levelbelow V₂.